bc: get rid of G.tty and G.flags

bc: get rid of G.tty and G.flags

G.flags == option_mask32

function                                             old     new   delta
bc_args                                               97     120     +23
dc_main                                               48      49      +1
bc_main                                               48      49      +1
bc_error                                              42      38      -4
bc_posix_error                                        65      60      -5
bc_read_line                                         325     317      -8
dc_lex_token                                         714     705      -9
bc_vm_run                                           2039    1942     -97
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/5 up/down: 25/-123)           Total: -98 bytes
   text	   data	    bss	    dec	    hex	filename
 987717	    485	   7296	 995498	  f30aa	busybox_old
 987619	    485	   7296	 995400	  f3048	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>